Fastening device

ABSTRACT

A fastening device for fastening a bolt to a fastening target object, wherein the bolt has a bolt head female screw section in an inner circumferential surface of a recess provided in an upper surface of a bolt head section. The fastening device includes a tension rod having a rod male screw section engaging the bolt head female screw section; a drive socket circumferentially surrounding the tension rod and having a lower end section with a grasper for laterally grasping the bolt head section; a supporter circumferentially surrounding the drive socket and having a lower end section protruding downward beyond a lower end section of the drive socket; a sensor for sensing compression force acting on the supporter in the upward/downward direction; a first motor for rotating the tension rod around an axis extending in the upward/downward direction; and a second motor for rotating the drive socket around the axis.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional patent application of Ser. No. 17/048,807 filed onOct. 19, 2020, which is a National Phase of International ApplicationNo. PCT/JP2019/019250 filed on May 15, 2019, and claims priority ofJapanese Patent Application No. 2018-231252 filed on Dec. 11, 2018, thedisclosures of which are hereby incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a fastening device that fastens a boltto a fastening target object.

BACKGROUND ART

A bolt/nut fastener is used to assemble and fix a large number ofmechanical structures including an automobile as a representativeexample. Patent Literature 1 discloses a bolt having a male screwsection formed around the outer circumferential surface of the bolt headsection.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent No. 6,381,840

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to automate bolt fasteningoperation.

Solution to Problem

To achieve the object described above, the invention of the presentapplication relates, as a viewpoint, to (1) a fastening device forfastening a bolt to a fastening target object, the bolt so configuredthat a bolt head section male screw section is formed around a sidesurface of a bolt head section and an engagement receiving section isformed at a top surface of the bolt head section, the fastening deviceincluding a tension rod including a rod female screw section thatengages with the bolt head section male screw section, a bit that is sodisposed inside the tension rod as to be movable in an upward/downwarddirection, includes an engagement section that engages with theengagement receiving section, and rotates the bolt around an axisextending in the upward/downward direction with the engagement receivingsection engaging with the engagement section, a supporter that isdisposed in a position where the supporter circumferentially surroundsthe tension rod and has a lower end section protruding downward beyond alower end section of the tension rod, a sensor for sensing compressionforce acting on the supporter in the upward/downward direction, a firstmotor for rotating the bit around the axis, and a second motor forrotating the tension rod around the axis.

(2) The fastening device described in item (1) above, in which a boltshaft male screw section is formed around a shaft section of the bolt,and the rod female screw section and the bolt shaft male screw sectionhave the same screw pitch.

(3) The fastening device described in item (1) or (2) above, in which atubular holder is disposed inside the tension rod, and a magnet, the bitattracted by the magnet, and a spring that urges the magnet downward aredisposed in the holder.

(4) The fastening device described in any one of (1) to (3) above, inwhich the engagement receiving section is an insertion hole, and theengagement section is a lower end section of the bit inserted into theinsertion hole.

(5) The invention of the present application relates, as anotherviewpoint, to a fastening device for fastening a bolt to a fasteningtarget object, the bolt so configured that a bolt head section femalescrew section is formed in an inner circumferential surface of a recessprovided in an upper surface of a bolt head section, the fasteningdevice including a tension rod including a rod male screw section thatengages with the bolt head section female screw section, a drive socketthat is disposed in a position where the drive socket circumferentiallysurrounds the tension rod and has a lower end section including agrasper for laterally grasping the bolt head section, a supporter thatis disposed in a position where the supporter circumferentiallysurrounds the drive socket and has a lower end section protrudingdownward beyond a lower end section of the drive socket, a sensor forsensing compression force acting on the supporter in the upward/downwarddirection, a first motor for rotating the tension rod around an axisextending in the upward/downward direction, and a second motor forrotating the drive socket around the axis.

(6) The fastening device described in (5) above, in which a bolt shaftmale screw section is formed around a shaft section of the bolt, and therod male screw section and the bolt shaft male screw section have thesame screw pitch.

(7) The fastening device described in (5) or (6) above, in which aprojection is formed at a lower end section of the tension rod, and therod male screw section is formed around an outer circumferential surfaceof the projection.

Advantageous Effects of Invention

The configuration of above-mentioned item (1) of the present inventionallows automated tightening of a bolt so configured that a male screwsection is formed around the outer circumferential surface of a bolthead section. The configuration of above-mentioned item (5) of thepresent invention allows automated tightening of a bolt so configuredthat a bolt head section female screw section is formed in the innercircumferential surface of a recess in the top surface of a bolt headsection.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a fastening device (firstembodiment).

FIG. 2 is a cross-sectional view of a fastening target object and a bolt(first embodiment).

FIG. 3 is a perspective view of the bolt (first embodiment).

FIG. 4(a) is a perspective view of a bolt (variation).

FIG. 4(b) is a cross-sectional view of a fastening target object and abolt (variation).

FIG. 5 describes the action (first half) of the fastening deviceaccording to the first embodiment.

FIG. 6 describes the action (second half) of the fastening deviceaccording to the first embodiment.

FIG. 7 shows graphs illustrating the relationship among the angle ofrotation of the bolt, axial force acting on the bolt, and tensile forceacting on the bolt.

FIG. 8 is a cross-sectional view of a fastening device (secondembodiment).

FIG. 9 is a perspective view of part of a bolt (second embodiment).

FIG. 10 describes the action (first half) of the fastening deviceaccording to the second embodiment.

FIG. 11 describes the action (second half) of the fastening deviceaccording to the second embodiment.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a cross-sectional view showing a fastening device 100. Thefastening device 100 includes a first motor 10, a second motor 30, andan enclosure 50. The first motor 10 and the second motor 30 areindependent of each other and driven and controlled by a controller 5.That is, the controller 5 can control the first motor 10 and the secondmotor 30 independently of each other. The controller 5 can, for example,be a CPU (central processing unit).

The enclosure 50 includes a body 51 and a base 52, which has a recessedupper end fixed to the lower surface of the body 51. The body 51accommodates a first decelerator 11 and a second decelerator 31. Thefirst decelerator 11 includes an output shaft 11 a. The output shaft 11a has a tubular shape, and a power transmitter 12 is inserted into andfixed to a tubular section of the output shaft 11 a. The powertransmitter 12 rotates along with the output shaft 11 a. A lower endsection of the power transmitter 12 protrudes downward beyond the outputshaft 11 a, and the protruding section extends toward the interior of atubular bit holder 13, which extends in the upward/downward direction,and is fixed. The rotational output produced by the output shaft 11 a istherefore transmitted to the bit holder 13 via the power transmitter 12to allow rotation of the output shaft 11 a and the bit holder 13integrated with each other.

The output shaft 11 a and the bit holder 13 are disposed in a roughlycoaxial manner and so supported as to be rotatable relative to a bearing14. A magnet 13 a is provided in the bit holder 13 and attracts a bit 13b for rotating a bolt. The bit 13 b extends downward and extends beyondthe lower end section of the bit holder 13. In the configurationdescribed above, rotating the bit holder 13 allows the bit 13 b torotate around an axis extending in the upward/downward direction.

The magnet 13 a and the bit 13 b are so accommodated in the tubular bitholder 13 as to be slidably movable in the upward/downward direction,and a spring 13 c is interposed between the magnet 13 a and the powertransmitter 12. The spring 13 c is disposed along the inner wall of thebit holder 13. When the magnet 13 a slides in the direction in which themagnet 13 a approaches the power transmitter 12, the spring 13 c isloaded in the compressed direction. The configuration described aboveallows the bit 13 b to move in the upward/downward direction in such away that the bit 13 b follows a bolt 70, which will be described later.

The bit 13 b wears due to a load exerted thereon when the bolt rotates.The configuration in which the bit 13 b is attracted and fixed to themagnet 13 a as in the present embodiment allows the bit 13 b to bereadily exchanged when the bit 13 b has worn.

The rotational force produced by the second motor 30 is transmitted to atension rod 15 via the second decelerator 31, a pinion gear 32, and anidle gear 33. That is, a drive gear 151, which is formed around theouter circumferential surface of the tension rod 15, engage with theidle gear 33, and the second motor 30 can be operated to rotate thetension rod 15 around an axis extending in the upward/downwarddirection.

Bearings 34 are disposed to receive loads acting on the pinion gear 32and the idle gear 33 when the shaft of the second motor 30 rotates. Theidle gear 33 is rotationally driven around an idle shaft 35 extending inthe upward/downward direction.

The tension rod 15 includes a tension accommodator 15 a foraccommodating the bit holder 13 and the bearing 14. A gap that allowsrotation of the tension rod 15 is formed between the bit holder 13 andthe tension rod 15. The tension rod 15 is rotatably supported bybearings 16 and a thrust bearing 17.

A tension rod female screw section 15 a 1 is formed in the inner radialsurface of a lower end section of the tension rod 15. The tension rodfemale screw section 15 a 1 engages with a bolt male screw sectionformed around the bolt head section when the bolt is pulled. Theengagement will be described later in detail.

A supporter 18 includes a supporter accommodator 18 a for accommodatingthe tension rod 15. The upper end section of the supporter 18 is incontact with the thrust bearing 17, and the lower end section of thesupporter 18 extends downward beyond the lower end section of thetension rod 15. Therefore, when the entire fastening device 100 lowerstoward a fastening target object H, the lower end section of thesupporter 18 comes into contact with the fastening target object H. Agap that allows rotation of the tension rod 15 is formed between thetension rod 15 and the supporter 18.

A sensor 181 is provided on the side facing the upper end of thesupporter 18. The sensor 181 includes a straining element 181 a and astrain gauge 181 b. The straining element 181 a has a cylindrical shapehaving a reduced-diameter intermediate section. The strain gauge 181 bis attached to the intermediate section of the straining element 181 a.

In the process of pulling the bolt, the straining element 181 a receivesupward force having the same magnitude as that of tensile forcetransmitted from the supporter 18 and is therefore distorted in thecompression direction within an elastic deformation range of thestraining element 181 a. The strain gauge 181 b changes in terms ofresistance in accordance with the strain of the straining element 181 aand changes output voltage to be outputted to the controller 5. Thecontroller 5 calculates the tensile force acting on the screw section ofthe bolt based on the amount of change in the output voltage from thestrain gauge 181 b. The sensor 181 is not limited to the combination ofthe straining element 181 a and the strain gauge 181 b and can beanother sensor capable of detecting the tensile force acting on thebolt.

The bolt to be tightened by the fastening device 100 according to thepresent embodiment will next be described with reference to FIGS. 2 and3 . FIG. 2 is a cross-sectional view of the bolt and the fasteningtarget object, and FIG. 3 is a perspective view of the bolt. The bolt 70is a bolt with a hexagonal hole and is formed of a bolt shaft section 71and a bolt head section 72. The bolt shaft section 71 has a male screwformed therearound. The fastening target object H (in other words,workpiece) is formed of fastening target objects H1 and H2, which arelayered on each other in the upward/downward direction, and bolt holesH1 a and H2 a are formed in the fastening target objects H1 and H2,respectively.

The bolt 70 is inserted into the bolt holes H1 a and H2 a, and a nut 80is caused to engage with the bolt shaft section 71 protruding downwardbeyond the end surface of the fastening target object H (H2). The bolt70 is thus fastened to the fastening target object H. It is, however,noted that the invention of the present application is applicable to afastener formed only of the bolt 70 with no nut 80. In this case,forming a female screw section that engages with the male screw sectionof the bolt shaft section 71 in the circumferential surface of the bolthole H2 a allows the bolt 70 to be fastened to the fastening targetobject H.

An insertion hole 72 b having a hexagonal shape (corresponding toengagement receiving section) is formed in the top surface of the bolthead section 72. A lower end section (corresponding to engagementsection) of the bit 13 b can be inserted into the insertion hole 72 band rotated to fasten the bolt 70 to the fastening target object H. Abolt head section male screw section 72 a is continuously formed aroundthe side surface of the bolt head section 72 in the circumferentialdirection with no interruptions. Rotating the tension rod 15 allows thebolt 70 to be pulled with the tension rod female screw section 15 a 1engaging with the bolt head section male screw section 72 a.

Now, let S1 be the area over which the supporter 18 and the fasteningtarget object H are in contact with each other and S2 be the area overwhich the bolt head section 72 and the fastening target object H are incontact with each other, and it is desirable that the contact areas S1and S2 are equal to each other. The seat surface of the bolt headsection 72 and the supporter 18 exert surface pressure on the fasteningtarget object H. Therefore, when the contact areas S1 and S2 differ fromeach other (that is, when difference in surface pressure is present),the amount of deformation of the fastening target object H undesirablydiffers from the amount of deformation in actual bolt fastening.Further, when the difference in surface pressure excessively increases,the supporter 18 is likely to deform and damage the fastening targetobject H, and the difference in the amount of deformation is likely tolower the fastening precision.

In the present embodiment, the insertion hole 72 b has a hexagonalshape, but not necessarily in the present invention, and the insertionhole 72 b may have an octagonal or any other polygonal shape. Theinvention of the present application is also applicable to a hexagonalbolt having a hexagonal bolt head section. In this case, an insertionhole 72 b′ may be formed in the top surface of a bolt head section 72′,and bolt head section male screw sections 72 a′ may be intermittentlyformed at curved sections of the side surface of the bolt head section72′, as shown in FIG. 4(a). Further, the bolt head section male screwsection 72 a (72 a′) is not necessarily formed across the bolt headsection 72 (72′) from the upper end to the lower end thereof and mayinstead be formed only part of the section from the upper end to thelower end as long as an engagement length necessary for the rotation ofthe tension rod 15 can be ensured.

In the present embodiment, the bit 13 b is inserted into the insertionhole 72 b formed in the bolt head section 72 to rotate the bolt 70, butnot necessarily in the present invention. Instead, a protrusion 72 c(corresponding to engagement receiving section) may be formed at thebolt head section 72 (72′), and the protrusion 72 c may be inserted intoa recess (corresponding to engagement section) that is not shown butformed in the lower end section of the bit 13 b to rotate the bolt 70,as shown in FIG. 4(b). That is, the invention of the present applicationis widely applicable to bolts each having a bolt head section male screwsection formed around the side surface of the bolt head section and anengagement receiving section for engagement with the engagement sectionof the bit formed at the bolt head section. The invention of the presentapplication is further applicable to a flanged bolt having a flangeformed around the bolt head section.

The action of the fastening device 100 will next be described withreference to FIGS. 5 and 6 , which are descriptive diagrams of theaction. It is now assumed that the bolt 70 is temporarily attached tothe fastening target object H, and that the bolt head section 72 islocated in a temporal attachment position separate from the uppersurface of the fastening target object H in an initial state (see FIG.5(a)). It is further assumed that the bit 13 b is inserted into theinsertion hole 72 b of the bolt head section 72. It is assumed that thecontrol described below is performed by the controller 5 unlessotherwise stated.

When the first motor 10 is operated, the first decelerator 11, theoutput shaft 11 a, the power transmitter 12, and the bit holder 13rotate, and the bit 13 b held by the bit holder 13 rotates in thedirection indicated with the arrow K1. When the bit 13 b rotates, thebolt 70 screws downward. The fastening device 100 is so supported as tobe allowed to lower by its own weight relative to a support section (notshown), and the entire fastening device 100 therefore lowers with thebit 13 b rotating. As a result, the tension rod female screw section 15a 1 and the bolt head section male screw section 72 a become engageablewith each other (see FIG. 5(b)).

When the bolt 70 further rotates and the bolt head section 72 is seatedon the upper surface of the fastening target object H, the force that isreceived by the first motor 10 and attempts to rotate the shaft of thefirst motor 10 abruptly increases, resulting in an abrupt increase infeedback torque (see FIG. 5(c)). The controller 5 senses based on thefeedback torque produced by the first motor 10 that the bolt 70 has beenseated and stops the operation of the first motor 10. The shaft of thefirst motor 10 is then reversely rotated in the direction indicated bythe arrow K2 to move the bolt head section 72 back to the positionslightly separate from the upper surface of the fastening target objectH (see FIG. 5(d)).

Thereafter, the operation of the first motor 10 is stopped, and thesecond motor 30 is operated. When the second motor 30 is operated, theentire fastening device 100 moves downward with the tension rod 15rotating in the direction indicated by the arrow K1, so that the tensionrod female screw section 15 a 1 and the bolt head section male screwsection 72 a engage with each other (see FIG. 6(e)). In this process,the bit 13 b slides upward in the interior of the bit holder 13 whileresisting the elastic force provided by the spring 13 c.

When the tension rod 15 is further caused to screw downward whilerotating, the lower end section of the supporter 18 comes into contactwith the upper surface of the fastening target object H, so that thefastening device 100 stops lowering. Since the supporter 18 is incontact with the fastening target object H, an attempt to further rotatethe tension rod 15 in the direction indicated with the arrow K1 cannotcause the tension rod 15 to screw downward. In this process, since thebit 13 b inserted into the insertion hole 72 b of the bolt head section72 prevents the bolt 70 from rotating, the tension rod 15 exerts a loadin the pulling direction (that is, upward) on the bolt 70. However,since the supporter 18 is in contact with the fastening target object H,rotation of the tension rod 15 cannot move the bolt 70 in the pullingdirection. As a result, tensile force P acts on the nut 80 and thesupporter 18 (see FIG. 6(f)). Since the upper end section of thesupporter 18 is in contact with the thrust bearing 17, the thrustbearing 17 and the fastening target object H compress the supporter 18,and the compression force is detected as tensile force P with the sensor181.

A target value of the tensile force P is now defined as target tensileforce (10 kN, for example). To prevent overshooting operation of thesecond motor 30, it is desirable to decelerate the rotation of the shaftof the second motor 30 as the tensile force P approaches the targettensile force and stop the operation of the second motor 30 when thetensile force P reaches the target tensile force (corresponding tosecond step). The target tensile force is also target axial force actingon the bolt 70.

The shaft of the first motor 10 is then caused to rotate again in thedirection indicated with the arrow K1 with the tensile force P appliedto cause the bolt head section 72 to be seated on the fastening targetobject H (corresponding to third step). It is desirable that the tensionrod female screw section 15 a 1 and the male screw formed around thebolt shaft section 71 have the same screw pitch. If the screw pitchesdiffer from each other, the rotation control performed on the bit 13 b(that is, drive control performed on first motor 10) and the rotationcontrol performed on the tension rod 15 (that is, drive controlperformed on second motor 30) need to be performed simultaneously,resulting in cumbersome control. On the other hand, when the screwpitches are equal to each other, the bolt is tightened only by rotationof the bit 13 b, resulting in no cumbersome synchronous control.

When the bolt head section 72 comes into contact with the fasteningtarget object H, the tensile force P starts decreasing, whereas axialforce F acting on the bolt 70 increases (see FIGS. 6(g) and 7). Thederivative of the tensile force P, dP/dθ, is regularly monitored, andthe rotation of the bit 13 b is stopped when the behavior of dP/dθchanges from an unstable curve (nonlinear region) to a straight line(corresponding to fourth step). The term “dθ” corresponds to the angleof rotation of the bolt 70.

Finally, the shaft of the second motor 30 is rotated in the directionopposite the direction at the time of bolt tightening to retract thefastening device 100 from the fastening target object H and cause thebolt head section 72 and the tension rod 15 to disengage from each other(see FIG. 6(h)). At this point, the axial force F acting on the bolt 70shows a value close to the target tensile force, whereby precise axialforce F can be provided.

In the present embodiment, the operation of tightening the bolt 70 isstopped when the axial force becomes greater than the target axial forceF to cause the bolt head section 72 and the tension rod 15 to disengagefrom each other, as shown in FIG. 7 . When the operation of tighteningthe bolt 70 is stopped immediately after the bolt 70 is seated on thefastening target object H to cause the bolt head section 72 and thetension rod 15 to disengage from each other, the axial force decreasesby the amount corresponding to the elastic deformation of the bolt 70and the fastening target object H. A decrease σ representing thedecrease described above corresponds to the difference between the axialforce produced when the behavior of dP/dθ changes from a curve to astraight line and the target axial force F. It is, however, noted thatthe tensile force P may be set in advance at a value greater by thedecrease σ in expectation of the decrease in the axial force after thebolt 70 is seated, and the operation of tightening the bolt 70 may bestopped immediately after the bolt 70 is seated. In this case, when thebolt head section 72 and the tension rod 15 are caused to disengage fromeach other, the axial force acting on the bolt 70 decreases toward thetarget axial force F.

Second Embodiment

FIG. 8 is a cross-sectional view of a fastening device 200. FIG. 9 is aperspective view of part of the bolt. A bolt 90 includes a bolt headsection 91 and a bolt shaft section 92. A bottomed, tubular head sectionopening 91 a (corresponding to recess) is formed in the top surface ofthe bolt head section 91, and a head section female screw section 91 bis formed in the inner circumferential surface of the head sectionopening 91 a. A male screw 92 a is formed around the bolt shaft section92. The bolt 90 is fastened along with a nut 93 (see FIGS. 10 and 11 ),which will be described later, to the fastening target object H. Thefastening target object H is the same as that in the first embodimentand will therefore not be described.

The fastening device 200 includes a first motor 210, a second motor 220,and an enclosure 223. The first motor 210 and the second motor 220 areindependent of each other and driven and controlled by a controller 9.That is, the controller 9 can control the first motor 210 and the secondmotor 220 independently of each other. The controller 9 can, forexample, be a CPU (central processing unit).

The enclosure 223 includes a body 223 a and a base 223 b, which has arecessed upper end fixed to the lower surface of the body 223 a. Thebody 223 a accommodates a first decelerator 211 and a second decelerator221. A tension rod 201 for rotating the bolt 90 protrudes beyond thelower surface of the enclosure 223. A columnar protrusion 201 a having acolumnar shape (corresponding to projection) is provided at the lowerend section of the tension rod 201, and a rod male screw section 201 bare formed around the outer circumferential surface of the columnarprotrusion 201 a.

The rotational force produced by the first motor 210 is transmitted tothe tension rod 201 via the first decelerator 211, and the tension rod201 rotates around an axis extending in the upward/downward direction.Rotating the tension rod 201 with the rod male screw section 201 b ofthe tension rod 201 and the head section female screw section 91 bengaging with each other allows the bolt 90 to be tightened. Thefastening device 200 is so supported as to be allowed to lower by itsown weight relative to a support section (not shown). Therefore, whenthe bolt 90 screws toward the fastening target object H, the entirefastening device 200 lowers along with the bolt 90.

The second motor 220 is located next to the first motor 210 in aside-by-side fashion and connected to a second decelerator 221 locatedimmediately below the second motor 220. The second decelerator 221decelerates the rotation inputted from the second motor 220 and outputsthe decelerated rotation via an output shaft 229. The output shaft 229is so supported as to be rotatable relative to a bearing 222.

The rotational force produced by the second motor 220 is transmitted toa drive socket 202 via the second decelerator 221, the output shaft 229,a pinion gear 227, and an idle gear 228. That is, a drive gear 316,which is formed around the outer circumferential surface of the drivesocket 202, engage with the idle gear 228, and the second motor 220 canbe operated to rotate the drive socket 202 around an axis extending inthe upward/downward direction.

Bearings 225 are disposed to receive loads acting on the pinion gear 227and the idle gear 228 when the shaft of the second motor 220 rotates.The idle gear 228 is rotationally driven via an idle shaft 226 extendingin the upward/downward direction.

The drive socket 202 protrudes beyond the lower surface of the enclosure223. The drive socket 202 is so supported as to be rotatable relative toa pair of upper and lower bearings 217 a, 217 b and a bearing 312. Aretaining ring 216 is provided to prevent the upper bearing 217 a fromfalling off.

The drive socket 202 has a tubular shape and accommodates the tensionrod 201. A bolt head section grasper 202 a, which protrudes inward inthe radial direction, is formed at a lower end section of the drivesocket 202. The inner radial surface of the bolt head section grasper202 a has a shape corresponding to the side surface of the bolt headsection 91 (that is, hexagonal shape in the present embodiment).Rotating the drive socket 202 with the bolt head section grasper 202 afit to the bolt head section 91 allows the bolt 90 to screw relative tothe fastening target object H. An attempt to rotate the bolt 90 via thetension rod 201 with the drive socket 202 being stationary cannot rotatethe bolt 90 because the bolt head section grasper 202 a grasps the bolthead section 91.

A supporter 203 includes a supporter accommodator 203 a foraccommodating the drive socket 202. The upper end section of thesupporter 203 is in contact with the bearing 217 b, and the lower endsection of the supporter 203 extends downward beyond the lower endsection of the drive socket 202. Therefore, when the entire fasteningdevice 200 lowers toward the fastening target object H, the lower endsection of the supporter 203 comes into contact with the fasteningtarget object H. A gap that allows the rotation of the drive socket 202is formed between the drive socket 202 and the supporter 203.

Now, let S1 be the area over which the supporter 203 and the fasteningtarget object H are in contact with each other and S2 be the area overwhich the bolt head section 91 and the fastening target object H are incontact with each other, and it is desirable that the contact areas S1and S2 are equal to each other. The seat surface of the bolt headsection 91 and the supporter 203 exert surface pressure on the fasteningtarget object H. Therefore, when the contact areas S1 and S2 differ fromeach other (that is, when difference in surface pressure is present),the amount of deformation of the fastening target object H undesirablydiffers from the amount of deformation in actual bolt fastening.Further, when the difference in surface pressure excessively increases,the supporter 203 is likely to deform and damage the fastening targetobject H, and the difference in the amount of deformation is likely tolower the fastening precision.

A sensor 21 includes a straining element 213 and a strain gauge 214. Thestraining element 213 has a cylindrical shape having a reduced-diameterintermediate section. The straining element 213 can be made of metal.The straining element 213 is disposed radially outside the tension rod201 and between the decelerator 211 and the drive socket 202. The upperportion of the straining element 213 is pressed by the decelerator 211via a thrust bearing 212. A lower portion of the straining element 213is supported by the drive socket 202 via a thrust bearing 215. Thestrain gauge 214 is attached to the intermediate section of thestraining element 213. The sensor 21 may instead be provided in thesupporter 203.

In the process of pulling the bolt 90, the straining element 213receives upward force having the same magnitude as that of tensile forcetransmitted from the supporter 203 and is therefore distorted in thecompression direction within an elastic deformation range of thestraining element 213. The strain gauge 214 changes in terms ofresistance in accordance with the strain of the straining element 213and changes output voltage to be outputted to the controller 9. Thecontroller 9 calculates the tensile force acting on the screw section ofthe bolt 90 based on the amount of change in the output voltage from thestrain gauge 214. Thrust bearings 212 and 215, which are provided on theupper and lower sides of the sensor 21, have the function of suppressingtransmission of turbulent force received from an object other than thesupporter 203 to the sensor 21. The sensor 21 is not limited to thecombination of the straining element 213 and the strain gauge 214 andcan be another sensor capable of detecting the tensile force acting onthe bolt 90.

The action of the fastening device 200 will next be described withreference to FIGS. 10 and 11 , which are descriptive diagrams of theaction. It is now assumed that the bolt 90 is temporarily attached tothe fastening target object H, and that the bolt head section 91 islocated in a temporal attachment position separate from the uppersurface of the fastening target object H in an initial state (see FIG.10(a)). It is further assumed that the bolt head section grasper 202 aof the drive socket 202 grasps an upper end section of the bolt headsection 91. It is still further assumed that the columnar protrusion 201a of the tension rod 201 is in contact with the upper end of the headsection female screw section 91 b of the bolt head section 91. It isstill further assumed that the control described below is performed bythe controller 9 unless otherwise stated.

When the second motor 220 is operated, the second decelerator 221, theoutput shaft 229, the pinion gear 227, the idle gear 228, and the drivegear 316 rotate, and the bolt 90 grasped by the grasper 202 a of thedrive socket 202 rotates in the direction indicated by the arrow K1 (inother words, clockwise direction in plan view). When the drive socket202 rotates, the bolt 90 screws downward. The fastening device 200 is sosupported as to be allowed to lower by its own weight relative to asupport section (not shown), and the entire fastening device 200therefore lowers with the drive socket 202 rotating.

When the drive socket 202 further rotates the bolt 90 in the directionindicated by the arrow K1 and the bolt head section 91 is seated on theupper surface of the fastening target object H, the force that isreceived by the second motor 220 and attempts to rotate the shaft of thesecond motor 220 abruptly increases, resulting in an abrupt increase inthe feedback torque. The controller 9 senses based on the feedbacktorque produced by the second motor 220 that the bolt 90 has been seatedand stops the operation of the second motor 220. FIG. 10(b) shows thestate immediately after the operation of the second motor 220 isstopped.

After the operation of the second motor 220 is stopped, the second motor220 is so operated that the shaft thereof rotates in the oppositedirection to rotate the drive socket 202 in the direction indicated withthe arrow K2 (in other words, counterclockwise direction in plan view).When the drive socket 202 is rotated in the direction indicated with thearrow K2, the bolt 90 screws upward, and the tension rod 201, the drivesocket 202, and the supporter 203 integrally move upward. The bolt headsection 91 thus moves to a position slightly separate from the uppersurface of the fastening target object H (in other words, tension roddrive start position). FIG. 10(c) shows the state immediately after thebolt 90 reaches the tension rod drive start position.

Thereafter, the operation of the second motor 220 is stopped, and thefirst motor 210 is operated. When the first motor 210 is operated, thetension rod 201 screws toward the bottom surface of the head sectionopening 91 a of the bolt head section 91 while rotating in the directionindicated by the arrow K1, and the drive socket 202 and the supporter203 lower along with the tension rod 201 toward the fastening targetobject H. FIG. 11(d) shows the state in a halfway position where thetension rod 201, the drive socket 202, and the supporter 203 lowertoward the fastening target object H. The position of the bolt 90 doesnot change in the course of transition from the step shown in FIG. 10(c)to the step shown in FIG. 11(d).

When the tension rod 201 further screws, the lower end section of thesupporter 203 comes into contact with the upper surface of the fasteningtarget object H, so that the fastening device 200 stops lowering. FIG.11(e) shows the state immediately after the supporter 203 comes intocontact with the upper surface of the fastening target object H. Sincethe supporter 203 is in contact with the fastening target object H, anattempt to further rotate the tension rod 201 in the direction indicatedwith the arrow K1 cannot cause the tension rod 201 to screw downward.

In this process, since the grasper 202 a of the drive socket 202prevents the bolt 90 from rotating, the tension rod 201 exerts a load inthe pulling direction (that is, upward) on the bolt 90. However, sincethe supporter 203 is in contact with the fastening target object H,rotation of the tension rod 201 cannot move the bolt 90 in the pullingdirection. As a result, tensile force P acts on the nut 93 and thesupporter 203 (see FIG. 11(e)). In this process, the members (such asstraining element 213) sandwiched between the thrust bearing 212 and thesupporter 203 are compressed, and the compression force is detected astensile force P with the strain gauge 214.

A target value of the tensile force P is now defined as target tensileforce (10 kN, for example). To prevent overshooting operation of thefirst motor 210, it is desirable to decelerate the rotation of the shaftof the first motor 210 as the tensile force P approaches the targettensile force and stop the operation of the first motor 210 when thetensile force P reaches the target tensile force. The target tensileforce is also target axial force acting on the bolt 90.

The shaft of the second motor 220 is then caused to rotate again in thedirection indicated with the arrow K1 with the tensile force P appliedto cause only the bolt 90 out of the tension rod 201, the drive socket202, the supporter 203, and the bolt 90 to screw downward. In otherwords, the bolt 90 moves in the direction in which the bolt 90 is pulledout of the tension rod 201. When the bolt 90 further screws downward,the bolt head section 91 is seated on the fastening target object H. Itis desirable that the head section female screw section 91 b and themale screw 92 a of the bolt shaft section 92 have the same screw pitch.If the screw pitches differ from each other when performing from thestep shown in FIG. 11(e) to the step shown in FIG. 11(f), the rotationcontrol performed on the tension rod 201 (that is, drive controlperformed on first motor 210) and the rotation control performed on thedrive socket 202 (that is, drive control performed on second motor 220)need to be performed simultaneously, resulting in cumbersome control. Onthe other hand, when the screw pitches are equal to each other, the bolt90 is tightened only by rotation of the drive socket 202, resulting inno cumbersome control.

When the bolt head section 91 comes into contact with the fasteningtarget object H, the tensile force P starts decreasing, whereas axialforce F acting on the bolt 90 increases (see FIGS. 11(f) and 7). Thederivative of the tensile force P, dP/dθ, is regularly monitored, andthe rotation of the drive socket 202 is stopped when the behavior ofdP/dθ changes from an unstable curve (nonlinear region) to a straightline. The term “dθ” corresponds to the angle of rotation of the bolt 90.

Finally, the shaft of the first motor 210 is rotated in the directionopposite the direction at the time of bolt tightening to retract thetension rod 201 to a position above the bolt head section 91 and causethe drive socket 202 and the bolt head section 91 to disengage from eachother. The bolt 90 is thus removed from the fastening device 200. Atthis point, the axial force F acting on the bolt 90 shows a value closeto the target tensile force, whereby precise axial force F can beprovided.

(Example of Other Usage of Fastening Device 100)

In the embodiments described above, the fastening device 100 (200) isused to newly tighten the bolt 70 (90), but not necessarily in thepresent invention, and the fastening device 100 (200) may be used toretighten the existing bolt 70 (90) determined to have insufficientaxial force in an axial force detection step. For example, when boltsare successively fastened to a flange-shaped multi-axis workpiece(fastening target object), the axial force acting on a bolt fastened inan early stage of the fastening process decreases in some cases. In suchcases, after the fastening device 100 (200) fastens the bolt, the axialforce acting on the bolt can be detected by pulling the bolt. An axialforce detection theory is described in Japanese Patent No. 4,028,254 andwill therefore not be described in detail. When it is determined thatthe axial force is insufficient as a result of the axial forcedetection, the fastening device 100 (200) can perform retightening tomanage the axial force with higher reliability. Moreover, the fasteningdevices 100 (200) according to the embodiments of the present inventioncan be arranged (in the form of a matrix, for example) andsimultaneously tighten a plurality of bolts 70 (90). For example, evenin the case of a multi-axis workpiece, such as bolts for a cylinder headof an engine, the fastening devices 100 (200) can perform simultaneoustightening, whereby stable axial force is provided across the entireproduct, resulting in improvement in the quality of the product.

REFERENCE SIGNS LIST

-   5, 9: Controller-   10, 210: First motor-   11, 211: First decelerator-   11 a: Output shaft-   12: Power transmitter-   13: Bit holder-   13 a: Magnet-   13 b: Bit-   13 c: Spring-   14: Bearing-   15, 201: Tension rod-   15 a 1: Tension rod female screw section-   18, 203: Supporter-   30, 220: Second motor-   70: Bolt-   71: Bolt shaft section-   72: Bolt head section-   72 a: Bolt head section male screw section-   72 b: Insertion hole-   100: Fastening device-   200: Fastening device

The invention claimed is:
 1. A fastening device for fastening a bolt toa fastening target object, the bolt being so configured that a bolt headsection female screw section is formed in an inner circumferentialsurface of a recess provided in an upper surface of a bolt head section,the fastening device comprising: a tension rod including a rod malescrew section that engages with the bolt head section female screwsection; a drive socket that is disposed in a position where the drivesocket circumferentially surrounds the tension rod and has a lower endsection including a grasper for laterally grasping the bolt headsection; a supporter that is disposed in a position where the supportercircumferentially surrounds the drive socket and has a lower end sectionprotruding downward beyond the lower end section of the drive socket; asensor for sensing compression force acting on the supporter in theupward/downward direction; a first motor for rotating the tension rodaround an axis extending in the upward/downward direction; and a secondmotor for rotating the drive socket around the axis, wherein a boltshaft male screw section is formed around a shaft section of the bolt,and the rod male screw section and the bolt shaft male screw sectionhave a same screw pitch.
 2. The fastening device according to claim 1,wherein a projection is formed at a lower end section of the tensionrod, and the rod male screw section is formed around an outercircumferential surface of the projection.